I got befuddled by the "AI servo not locking focus" a few days ago when I was focusing and recomposing, and kept taking one fuzzy image after another. It felt like witchcraft, that the image would go all blurry on me just as I recomposed to take the picture (I was using BBF but foolishly kept my finger on the BB) until I realized that I was on AI servo.

I am still trying to understand why it is not guaranteed to get a sharp picture with AI server for stationary objects with shallow dof as long as light is enough. But there won't be any focus confirmation beep. Considering there is no AF lock with AI Servo, it is not possible to do focus and recompose with it right?. Do I need to always switch to one shot AF with focus and recompose.Thanks

I am still trying to understand why it is not guaranteed to get a sharp picture with AI server for stationary objects with shallow dof as long as light is enough. But there won't be any focus confirmation beep. Considering there is no AF lock with AI Servo, it is not possible to do focus and recompose with it right?. Do I need to always switch to one shot AF with focus and recompose.Thanks

If you want to focus and recompose using ai servo then you need to use back button focus. In order for it to work you need to go to the customize settings for the shutter button and deselect the "half press initiates autofocus" option.. That way you use your center point to focus using the back button then let go, recompose and push the shutter button.

I always set one of the custom modes this way along with the fast tracking AF settings you see posted here and on other websites. That way you don't have to fiddle around and make a lot of setting changes to switch back and forth.

Remember that the AF sensor in the camera has a low light threshold, typically EV -1 or -2 depending on the camera; this figure is quoted specifically for the center AF point with One-Shot AF. It's usually about 2 stops less than than with AI Servo AF, and even lower with off-center focusing points. Therefore, if maximum sensitivity for AF in low light is your priority, we strongly recommend One-Shot AF with the center focusing point.

I particularly liked this bit. I use off-centre focussing points a lot and did not realise that there was such a difference.

Correct....the reason Ai-Servo is not guaranteed sharp is that it never stops focusing as long as the AF on is engaged, whether at the shutter button or on the camera back. If you're shooting motion...you're focused on maintaining your focus point on the subject. If, on the other hand, you're shooting static, with Ai Servo, you're really trying to use it like One-Shot AF...and but while you held the AF-on engaged, you may have moved off the intended subject point slightly. And there is the additional confounding factor that Ai Servo is apparently less light sensitive in practice than One-Shot. More sensitivity to light means more sensitivity to the point of intended focus.

Nevertheless, Ai Servo works exceedingly well across all of the newest Canon upper range, and is pretty darn good going back to the 7D, 1D Mk III and 1Ds III.

Reading across many, many photography forums, it becomes plainly evident that many, if not most focusing problems are more related to operator error in misusing or mis-understanding the focus system of their camera and setting it up correctly for the type of shooting being done.

Two things were clarified to me in the OP, the light sensitivity difference and the logical accuracy difference due to time to acquire focus between the two modes. I certainly am a bit more informed by this thread.

And btw, when you remove af-on from the shutter and using backbutton focus ( af-on) ai-servo acts like one-shot... (see custom settings)... just release af-on button and the focus will be locked...

I agreement that the focus Will be locked, but as explained in the beginning, ai-servo gives no garanty that THE object is in focus as you can always take the picture, while one-shot will only allow you to take a photo when the object is in focus (Beep, green light)